1. Field of the Invention
The present invention relates to a resealable safety vent for a sealed rechargeable battery having a plastic container and a sealed alkaline battery provided with the same safety valve.
2. Description of the Prior Art
Recently, many systems have been developed which use a rechargeable battery, taking advantage of the economy of repetitive use of the battery. This type of battery is used in portable electric and electronic appliances, electric vehicles and other mobile devices, uninterruptible power systems (UPS) and in many other kinds of power systems used for electrical storage for load leveling, etc. In order to make the power source unit lighter and smaller, the battery systems should have a higher energy density, longer life, higher safety, higher reliability, easier maintenance, higher operational economy and other performance characteristics. Among others, sealing of a battery container, is an indispensable item for alleviating the complexity of maintenance in, for example, liquid replenishment, which is required with the above mentioned performance items. Several technologies have been developed, to seal a battery without causing increase of the inner cell pressure. These include absorption by the negative electrode using the oxygen cycle reaction, the catalyst plug type, the auxiliary electrode and others. Among these technologies, the absorption by negative electrode is popularly used. The sealing of rechargeable batteries by means of absorption by the negative electrode is realized by a construction, where the capacity of the negative electrode is made larger than that of positive electrode. The capacity of the battery is adjusted so that only oxygen gas is generated from the positive electrode during charging, especially in the overcharge region, thus reducing the electrolyte to the lowest possible amount making the electrode group starved. In a battery thus constructed, the oxygen gas generated from the positive electrode during charging is reduced at the negative electrode and vanishes by absorption.
Gas pressure in the battery cell is maintained in equilibrium and the volume of the electrolyte is fixed without diminishing; therefore no replenishment is necessary. However, in many of the sealed rechargeable batteries, a resealable safety vent is provided in preparation for a possible abnormal increase of the internal pressure of the battery cell which could be caused by a reduced gas absorption capability of the negative electrode due to the passage of time or by an excessive charging current caused by a malfunction in a power supply device used for charging the battery. The resealable safety vent normally keeps a container sealed closing the inside of a battery cell from the ambient air, while permitting discharge of excessive gas generated within a battery cell under unusual conditions and seals again after the excess gas is discharged outside the cell.
Among the sealed rechargeable batteries, a rechargeable battery using an alkaline electrolyte in a nickel-cadmium system or nickel-metal hydride system where caustic alkali of the electrolyte consists mainly of potassium hydroxide does not directly participate in the charge (discharge electrochemical reactions) is preferred to a conventional lead-acid rechargeable battery system where sulfuric acid electrolyte also reacts. The alkaline battery has higher energy density, longer life and higher reliability and is being used more frequently.
In most of the small size sealed alkaline rechargeable batteries, a steel cell can is sealed at its top edge with a steel disk having a unitized polyamide gasket around its circumference. In the rechargeable batteries for use in an electric vehicle or electric storage, where by the nature of the application a large capacity, hence a large size is needed, a commonly used configuration for attaining safety and lightweight characteristics is that an electrode group consisting of a plurality of positive and negative plates with a separator in between and housed in a generally square or rectangular shaped alkali-resistive plastic container.
Representative examples of prior art resealable safety vents or valves for sealed alkaline rechargeable battery are described in the following discussion.
(1) A small size sealed alkaline nickel-cadmium rechargeable battery consisting of a steel cell container is a representative product of this category. Production of this type battery is the largest right now. This type of battery is described in the gazette of patent publications Japan, Jitsu-Kai Sho58-10146. As illustrated in FIG. 3, the battery is sealed with a configuration where a steel disc cover 34 having a through hole 32 at the centre surrounded by a circular extrusion 33 is welded to a steel cap 35 having a vent hole 37 which acts as the positive electrode. The through hole 32 is hermetically sealed by an elastic valve body 31 of synthetic rubber which is inserted between the steel disc cover 34 and the steel cap 35 in contact with the circular extrusion 33 cut into the elastic valve body 31. Normally, the top edge of a can 36, which houses an electrode group 39 made up of a positive and a negative plates with a separator in between, wound and soaked with electrolyte, is bent or crimped to be sealed with the cover 34 by placing a gasket 38 in between. If the pressure within the cell goes unusually high the high pressure gas pushes up the elastic valve body 31 placed on the through hole 32, and escapes through the vent hole 37. When the pressure within cell decrease the through hole 32 is closed again by the elastic valve body 31, and the cell is isolated from the ambient.
(2) A sealed alkaline rechargeable battery using a nickel metal hydride system or others comprising a plastic container, being the same type of battery as the present invention, is described below U.S. Pat. No. 5,258,242 being an example. One of embodiments of the U.S. patent is shown in FIG. 4. In FIG. 4, a resealable safety valve 40 has the construction where a safety valve body portion 41 having a through hole 41a at its bottom is screwed and attached to a safety valve cover 42 for example by threads as shown in the drawing, having a vent hole 42a. A space or passage 40a is thus created inside said safety valve 40 said space including means to hold a valve seat 44 having an axial part 44a inserted inside of a coil spring 43. An O-ring 45 made of synthetic rubber is disposed in a groove 44b in valve body portion 41.
The through hole 41a of safety valve body portion 41 is closed by the O-ring 45 which is pressed to the inner surface of the safety valve body 41 by the action of the coil spring 43. Attaching the resealable safety valve 40 to a battery container cell, seals and isolates the cell from ambient atmosphere. If the gas pressure within a cell goes higher than the specified operating pressure of the safety valve the excesss gas pressure pushes up the valve seat 44 and the O-ring 45 positioned on the through hole 41a of the safety valve body portion 41 and escapes through the vent hole 42a of the safety valve cover 42, via the space 40a. When the cell pressure decreases, the sealed condition is restored. Although in the above embodiment, an O-ring is used to keep the battery sealed, a synthetic rubber sheet may be used in place of the O-ring. Other structures where a safety valve body and cover of an electrolyte container are unitized or a safety valve is screwed onto a battery container and others have been put into service.
A number of problems remained unsolved, with resealable safety vents used for sealed alkaline rechargeable batteries having a plastic container: Improvement of this type of battery is needed for use as a power source for electric vehicles and other mobile devices.
The resealable safety vent illustrated in FIG. 3 is suitable, as already described, for a sealed alkaline rechargeable battery which employs a small steel cell container and has a high operating pressure (higher than 10 kgf/cm.sup.2) for the safety vent. For a sealed alkaline rechargeable battery with plastic container where the operating pressure of the safety vent needs to be low, however, the resealable safety vent as illustrated in FIG. 3 does not work. If the safety vent is set at a lower operating pressure with the construction remaining as it is, spread of the operating pressures is large, and the reliability deteriorates.
A resealable safety vent having a structure as illustrated in FIG. 4 is intrinsically applicable to a plastic container sealed alkaline rechargeable battery. However, when it is used in a power source for a moving apparatus such as an electric vehicle, the safety vent cover 42 becomes loose from being screwed to the safety valve body 41 because of external forces such as vibration, shock, acceleration, etc. and the operating pressure of the safety valve gradually tends to shift to a lower value.
Under such conditions, as the time the battery is in service increases and the charge-discharge cycles are repeated the vent operating pressure decreases further, and the gas generated during charging is discharged out of the cell before the electrical charge has reached a specified level, the quantity of electrolyte decreases and the internal resistance of the battery increases. The increased internal resistance results in an increased cell voltage during charging which can cause insufficient charging. Also lowered cell voltage during discharge causes deteriorated capacity and cycle life. Another problem intrinsic to a construction where a safety vent cover is fixed to a safety vent body by a threaded connection is that the safety vent cover might be unscrewed and loosened by a casual mistake of a user causing a change in the operating pressure of the safety vent resulting in the vent being unable to guarantee the operational reliability. Furthermore, in a sealed alkaline rechargeable battery where a resealable safety valve is made of parts screwed together and fixed to a cover of an electrolyte container, the adverse influence of external forces such as vibration, shock, etc. is significant and the reliability deteriorates further.